Variable geometry hysteresis control for a gas turbine engine

ABSTRACT

One embodiment of the present application is a gas turbine engine with a compressor that includes a variable geometry mechanism to vary working fluid flow area. A desired state of this mechanism is selected different than a current state to change the amount of flow area. The mechanism has a hysteresis band corresponding to a difference between increasing the flow area to reach the desired state and decreasing the flow area to reach the desired state. To control operation of the mechanism within this hysteresis band, a determination is made whether the desired state corresponds to a greater flow area or a lesser flow area relative to the current state.

BACKGROUND

The present invention relates to variable geometry mechanism control,and more particularly, but not exclusively, relates to control ofmechanical hysteresis in compressor variable geometry vanes of a gasturbine engine.

There continues to be an interest in utilizing variable geometrymechanisms in gas turbine engines—especially in compressors for thehigh-pressure ratio engine variety used for aircraft. Within thesemechanisms, one or more mechanical components can display hysteresis. Insome cases, it is desirable to account for such characteristics duringengine operation. Thus, there remains a need for further contributionsin this area of technology.

SUMMARY

One embodiment of the present invention includes a unique technique forcontrolling variable geometry of vanes in a gas turbine engine. Otherembodiments include unique apparatus, systems, methods, or devices tocontrol mechanical hysteresis of variable geometry vanes in a gasturbine engine, including but not limited to Compressor VariableGeometry (CVG). Further embodiments, forms, objects, features, aspects,benefits, and advantages of the present invention shall become apparentfrom the detailed description and drawings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of an aircraft including gas turbineengines;

FIG. 2 is a diagrammatic view of a control system for the aircraft ofFIG. 1 showing one of the gas turbine engines in section;

FIGS. 3 and 4 are each a partial sectional, diagrammatic viewsillustrating different stages of a variable geometry mechanism of thegas turbine engine taken along view line 3-3 of FIG. 2;

FIG. 5 is a graph illustrating mechanical hysteresis of the variablegeometry mechanism of FIG. 2;

FIG. 6 is a flow chart of one mode of operating the gas turbine engineof FIG. 2 to address mechanical hysteresis of the variable geometrymechanism; and

FIGS. 7-9 are control flow diagrams of selected aspects of the controlsystem of FIG. 2.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any alterations and further modificationsin the described embodiments, and any further applications of theprinciples of the invention as described herein are contemplated aswould normally occur to one skilled in the art to which the inventionrelates.

It has been discovered that movement of actuation linkages to change thesetting of a variable geometry vane mechanism can result in a positionalvariation that depends on whether a new setting is approached with apushing (compression) or pulling (tension) force. This positionalvariation corresponds to a mechanical hysteresis band. Typically, theposition of one part relative to another varies with the manner ofmovement because of play or slack in the interconnection of such parts.This play or slack may result from normal part tolerances, wear, and/orenvironmental conditions to name just a few possibilities.

In one embodiment of the present invention, a variable geometry vanearrangement has a range of positions from a most open position to a mostclosed position. The arrangement has a degree of hysteresis depending onwhether a desired position is approached from a more open position ormore closed position, where the resulting difference in approachcorresponds to a hysteresis band. To correct for this hysteresis, thedesired position is approached from the same direction whether startingthe transition from a more open position or a more closed position. Inthis context, a “direction” is either: (1) a closing directioncorresponding to a change from a more open position to less openposition or (2) an opening direction corresponding to a change from aless open position to a more open position; where the closing directionand the opening direction are opposite one another.

In one example, transition from a more open position to less openposition is performed by change in the closing direction, whiletransition from a less open position to a more open position isperformed by a change in the opening direction past the desired positionand reversing to approach the desired position from the closingdirection. This form tends to place the arrangement at the high end ofthe hysteresis band. In another form, transition from a less openposition to more open position is performed by change in the openingdirection, while transition from a more open position to a less openposition is performed by change in the closing direction past thedesired position and reversing to approach the desired position from theopening direction. Hysteresis correction according to this embodimentmay be performed only in response to certain specified conditions, orall of the time.

Referring to FIG. 1, another embodiment of the present invention isshown. In FIG. 1, an aircraft 5 is shown with two gas turbine engines15. The engines 15 are suitable to provide thrust to propel the aircraft5 in a standard manner. The engines 15 are of a turbofan type arrangedin a dual spool configuration as further described hereinafter.Additionally referring to FIG. 2, a diagrammatic view of a gas turbineengine system 10 is illustrated, with only one of the engines 15 shownto preserve clarity. It should be understood that the other engine 15 ofaircraft 5 is likewise arranged. The gas turbine engine 15 is coupled toa standard fuel regulator (not shown) and a controller 50. The gasturbine engine 15 is of a twin spool configuration defining a workingfluid pathway 21 with an inlet 22, an exhaust outlet 23, and a bypassduct 29. Adjacent inlet 22 is a hub 24 of the gas turbine engine 15. Alow pressure compressor 25 includes a fan 26 as a first stage.Furthermore, a low pressure compressor 25 is connected to a low pressureturbine 27 by an inner shaft 28 forming a low pressure spool 20 whichrotates about rotational axis R.

The gas turbine engine 15 also includes a high pressure spool or core30. Core 30 is situated along the working fluid pathway 21 between thelow pressure compressor 25 and the low pressure turbine 27. Core 30includes a high pressure compressor 35 connected to a high pressureturbine 37 by an outer shaft 38 which rotates about axis R and innershaft 28 of the low pressure spool 20. Between high pressure compressor35 and high pressure turbine 37 are combustors 39. The combustors 39 areprovided fuel from a fuel feed line (not shown) that is controllablysupplied by the fuel regulator (not shown). Fuel supplied by thisregulator to the combustors 39 is controlled by signals output by thecontroller 50.

The gas turbine engine 15 includes several members that are driven torotate by working fluid as the working fluid flows along the pathway 21.These rotating members include the fan 26, the compressors 25 and 35,the turbines 27 and 37, and correspondingly the low pressure spool 20and the core 30. The high pressure compressor 35 includes a plurality ofblades 36 a interleaved with a number of stators 36 b. At least some ofstators 36 b may be operable to rotate about a corresponding variablegeometry axis along a longitudinal axis that extends radially from therotational axis R to provide Compressor Variable Geometry (CVG) inresponse to control signals from the controller 50, as more fullydescribed hereinafter. The variable geometry axes are more specificallydescribed in connection with FIGS. 3 and 4 hereinafter.

The mechanical operation of the gas turbine engine 15 includes drawingair as a working fluid through the inlet 22, pressurizing it with thecompressors 25 and 35, mixing the pressurized air with fuel to provide acombustible fuel charge, and igniting the fuel charge to providecombustion within the combustors 39. The rapidly expanding gasesresulting from this combustion drive the turbines 27 and 37 whichextract power therefrom including the power needed to rotate thecompressors 25 and 35. The working fluid exits low pressure turbine 27through outlet 23 generating thrust in a direction opposite thedirection of the exiting exhaust gases. In one preferred embodiment, thegas turbine engine 15 is a type of AE3007 turbofan engine manufacturedby Rolls-Royce Corporation.

The thrust generated by the gas turbine engine 15 corresponds to fuelselectively provided over a fuel line by the fuel regulator (not shown),which modulates fuel flow provided to the combustors 39 in response tocontrol signals from the controller 50. Controller 50 provides thesecontrol signals in accordance with controller operating logic 52.

The controller 50 can include analog circuitry, digital circuitry, or acombination of these. As an addition or alternative to electroniccircuitry, controller 50 may include one or more other types ofcomponents or control elements. Controller 50 may be comprised of one ormore components configured as a single unit or if of a multicomponentform, as a number of separate units. For such multicomponent forms, thecontroller 50 can have one or more components remotely located relativeto the others, or otherwise have its components distributed. Controller50 can be based on one or more general purpose integrated circuit chips,semicustom circuitry such as a Field Programmable Gate Array (FPGA) orthe like, a fully customized circuit type, a combination of these, orsuch different type as would occur to those skilled in the art.Controller 50 and/or system 10 also includes control clocks; powersupplies; signal conditioners; filters; limiters; Analog-to-DigitalConverters (ADCs); Digital-to-Analog Converters (DACs); communicationports and/or interfaces; or other types of circuits, devices, operators,elements, or components as would occur to those skilled in the art toimplement the present invention.

The controller 50 may be software and/or firmware programmable, a statelogic machine, other operationally dedicated hardware, or a hybridcombination of any of these. Controller 50 can include one or morememory devices (not shown), including but not limited to thesolid-state, optical, and/or magnetic variety. Such memory can providestorage for programming instructions executable by controller 50 asapplicable to the controller type, and/or can be arranged forreading/writing of data in accordance with one or more program routines.It should be appreciated that controller 50 operates in accordance withthe control logic 52 to perform various routines, procedures, and thelike—including those operations and conditionals described hereinafter.This operating logic can be in the form of software programminginstructions, firmware, programming of a gate array and/or applicationspecific circuitry, other hard-wired logic/circuitry, or a combinationof these, just to name a few examples. In one embodiment common toaircraft applications of the AE3007 type of engine 15, controller 50 isprovided as a Full Authority Digital Engine Control (FADEC). For thisembodiment, the FADEC digitizes all signals received in an analog formatfor use as a corresponding variable or value by a computer program formof logic 52. In one variation of this embodiment, two FADECs are used ina dual redundant configuration (not shown) to provide enhancedreliability in the event one fails.

The controller 50 is responsive to a number of input signals. Forexample, in the depicted embodiment controller 50 receives throttleangle signal TLA from an aircraft cockpit thrust control throttle 60.TLA corresponds to a desired engine thrust. Other signals provided tocontroller 50 correspond to various environmental parameters, such asTotal Air Temperature TAT corresponding to the surrounding airtemperature as sensed by a sensor 71, Mach Number MN corresponding tothe rate of travel of the aircraft 5 with respect to the speed of soundas sensed by a sensor 72, pressure altitude PALT corresponding to thealtitude as sensed by a sensor 73. As shown, these three signals aresupplied as analog electrical signals to an Air Data Computer (ADC) 70which converts them to a digital signal form for input to controller 50.In another embodiment, an ADC and controller may be included in a commonunit. The controller 50 also receives four signals corresponding tovarious performance parameters of the gas turbine engine 15. Rotationalspeed of the low pressure spool 20, N1, is sensed by a sensor 81. T2.5is sensed by a sensor 82 and corresponds to temperature between thecompressors 25 and 35. N2 is sensed by a sensor 83 that corresponds torotational speed of the core 30. ITT is sensed by a sensor 84 andcorresponds to interturbine temperature—the temperature between theturbines 27 and 37. In one embodiment, signals N1, T2.5, N2 and ITT areprovided to the controller 50 as analog electrical signals which arethen converted into a digital format therein. Collectively, thesesignals are designated by a path 90. In other embodiments, more, fewer,or otherwise different input signals, corresponding sensors, and thelike may be utilized or may be absent as would occur to one skilled inthe art, and/or may or may not include an ADC.

In one arrangement, the system 10 uses mechanical fan speed N1, as aprimary feedback control signal. A varying thrust request signal isgenerated that corresponds to desired/requested thrust selected with thethrottle 74 as selectively corrected for environmental factors input asPALT, MN, and TAT. This thrust request signal can be further limited,filtered, and/or otherwise conditioned. The feedback signal issubtracted from the request signal, as optionally corrected/conditioned,to obtain a control error signal corresponding to N1 speed error (anegative feedback arrangement). This error signal may be further tested,limited, and/or otherwise conditioned to prevent an undesirabledegree/manner of operational change. The resulting fan speed errorsignal, with optional conditioning, is determined by the controller 50and sent to the fuel regulator to modulate the fuel supplied to theengine 15. Gas turbine engine 15 responds to fuel flow adjustments witha change in speed of the turbines 27 and 37. As turbine speed changes sodoes fan speed N1 due to the connecting shaft 28. The loop continuesfrom the gas turbine engine 15 with input signal Ni to controller 50 viapath 90. Thus, while N1 differs from the desired fan speed, acorresponding fuel control speed adjustment signal is sent to the fuelregulator by controller 50. Once this desired fan speed is reached,controller 50 stops adjusting fuel to change turbine speed. When fan 26is damaged or is performing as though it is damaged, an alternative orsynthetic source of N1 can be utilized as further explained in U.S. Pat.No. 5,622,045 (which is hereby incorporated by reference in itsentirety) and/or different operational arrangements could be utilized aswould occur to one skilled in the art.

Through the primary feedback control loop based on fan speed N1, thecore speed N2 is controlled. Further, there is a secondary control loopthat can influence fueling that is based N2—and in particular, generallyseeks to maintain a given N2 versus N1 relationship within acceptableoperating parameters. To provide better fuel efficiency, CVG istypically employed to reduce N2 below a level that would otherwise berequired. In turn, high pressure compressor efficiency is oftenimproved.

The variable geometry mechanism 60 is operatively coupled to controller50 and is responsive to control signals therefrom as generated withvariable geometry control logic 62. Mechanism 60 includes at least oneactuator 61 a coupled to a linkage 61 b. The linkage 61 b is operativelycoupled to stators 36 b. Actuator 61 a is responsive to control signalsfrom controller 50 to selectively adjust linkage 61 b. Besides actuator61 a and linkage 61 b, mechanism 60 also collectively includes thestators 36 b of engine 15. The stators 36 b are mechanically coupled tolinkage 61 b for selective repositioning as further described withreference to FIGS. 3 and 4. In FIGS. 3 and 4, two different positions ofstators 36 b are schematically illustrated corresponding to twodifferent states or settings of mechanism 60, and each corresponds to apartial, diagrammatic sectional view taken along section line 3-3 ofFIG. 2; where like reference numerals refer to like features.

In FIGS. 3 and 4, the working fluid pathway 21 is defined between anouter flow path wall 21 a and an inner flow path wall 21 b (defined byshaft 38). With respect to axis R, the stators 36 b and blades 36 aradially extend into the pathway 21 from wall 21 a and wall 21 b,respectively. Shaft 38 of compressor 35 is shown about shaft 28, whichalso is shown in section. Axis R is represented by crosshairs in FIGS. 3and 4 because it is perpendicular to the view plane thereof. Stators 36b each rotate about a corresponding axis that is generally perpendicularto axis R and radially extends therefrom parallel to the view plane.Axes P1 and P2 represent two such stator rotation axes; wherecorresponding axes of the remaining stators 36 b have been omitted topreserve clarity.

FIG. 3 represents a state 64 a of the stators 36 b and mechanism 60 thatcorresponds to one possible CVG setting/position. State 64 a definesworking fluid flow area A1 for pathway 21 between the interleaved blades36 a and stators 36 b. FIG. 4 represents a state 64 b of stators 36 band mechanism 60 that corresponds to a different CVG setting/position.State 64 b defines working fluid flow area A2 for pathway 21 between theinterleaved blades 36 a and stators 36 b. As can be observed bycomparing FIGS. 3 and 4, stators 36 b of state 64 a present a broaderprofile in FIG. 3 than stators 36 b of state 64 b in FIG. 4. As aresult, area A2 is greater than area A1, such that state 64 acorresponds to a more closed position of stators 36 b relative to state64 b. Conversely, state 64 b corresponds to a more open position ofstators 36 b relative to state 64 a. By varying the rotation of stators36 b, a range of positions from a most open state to a least open (mostclosed) state can be realized.

As can be observed in connection with FIG. 5 of U.S. Pat. No. 5,622,045and its accompanying text (previously incorporated by reference), onedesired procedure progressively opens CVG as rotational engine speedincreases (increasing the working fluid flow area); and progressivelycloses CVG as rotational engine speed decreases (decreasing the workingfluid flow area).

Referring further to the graph of FIG. 5 of the present application, thedegree to which CVG is opened is shown relative to time for system10—revealing a hysteresis window H with a corresponding hysteresis bandB. When changing CVG in a closing direction, as represented by arrow D1of FIG. 5, the stators 36 b of mechanism 60 are at the high side of bandB, as represented by the upper dotted line. In contrast, when changingCVG in an opening direction, as represented by arrow D2 of FIG. 5, thestators 36 h of mechanism 60 are at the low side of band B, asrepresented by the lower dotted line. As previously explained, thishysteresis has been discovered to be largely mechanical innature—resulting from linkage/interconnection slack or play, as mightresult from wear, environmental conditions, normal tolerances or thelike, just to name a few possibilities. The solid line between the highside and low side of band B corresponds to the observed positionfeedback designated as signal CVGFB, which is input to controller 50 asshown in FIG. 2. CVGFB is the same regardless of the actual CVG positionwithin band B.

While the effects of mechanical hysteresis can be computationallyaddressed, it is desirable under at least some circumstances to controlactual position within the hysteresis band. By way of nonlimitingexample, this kind of mechanical hysteresis accommodation could bedesired to reduce the chances of exceeding a performance limit, whichcannot otherwise be achieved and still maintain desired efficiency. FIG.6 provides a flowchart illustrating a procedure 100 that can beperformed with system 10. Procedure 100 can be performed on a periodicor aperiodic basis, on demand, and/or in response to one or moreconditions to account for mechanical hysteresis of mechanism 60. Withinthe procedure 100, hysteresis adjustment is conditioned on the degree towhich an operating limit of engine 15 is approached. Procedure 100 isimplemented in the variable geometry control logic 62 of controller 50,and is executed when a change in CVG from a current state to a targetstate is requested by another operating routine of system 10. Such acalling routine is typically implemented in the control logic 52executed by controller 50. After the procedure 100 description, onenonlimiting, more detailed implementation is described with reference tothe control flow diagrams of FIGS. 7-9.

The procedure 100 begins with a conditional 102 that tests whether thecore rotational speed (N2) is above a threshold. In one form, thisthreshold can be selected relative to a safe operating limit of core 30,and/or in accordance with one or more other criteria. If the test ofconditional 102 is false (negative), procedure 100 returns to thecalling routine. On the other hand, if the test of conditional 102 istrue (affirmative), procedure 100 continues with a conditional 104. Theconditional 104 tests if the desired or target state of mechanism 60 ismore open than the current state of mechanism 60. One implementation oftracking such relative changes of state is further described inconnection with FIG. 7 hereinafter.

If the result of the conditional 104 test is false (negative), thenprocedure 100 continues with an operation 106. In operation 106, aunidirectional transition is performed from the current state to thetarget state. This transition is in the closing direction correspondingto arrow D1 of FIG. 5. Accordingly, the actual position of mechanism 60is on the high side of band B. It should be appreciated that while amonotonic decrease in CVG “openness” could be used to perform operation106, it alternatively could include direction reversals that do not fallbelow the target state resulting in one or more local minima; and/or maybe implemented with time periods where no substantial change occurs,such as might result through implementation in multiple discrete steps.In one form, mechanism 60 approaches the target state in a decreasing,stepwise manner corresponding to descent of a staircase. After executionof operation 106, procedure 100 returns to the calling routine.

Returning to conditional 104, if the target state of mechanism 60 ismore open than the current state, corresponding to a “true”(affirmative) test result, procedure 100 continues with amultidirectional transition operation 110. As a minimum, operation 110includes two stages. In a stage 112, change in the opening direction isperformed from the current state to an interim state. This interim stateis more open than the target state, such that in stage 112 mechanism 60is adjusted to go past (overshoot) the target state in the openingdirection.

In a stage 114 of operation 110, adjustment direction of mechanism 60reverses to approach the target state from the interim state. As aresult, the target state is approached in a closing direction.Consequently, both the operation 106 and the operation 110 reach thetarget state from a change in the closing direction to consistentlyplace the target state at the high side of the hysteresis band B (FIG.5). It should be appreciated that in this embodiment of the presentapplication, such a consistent final approach to the target state issubject to the test of conditional 102; however, in other embodimentsthis approach to a target state from a common direction regardless ofwhether the target state is more/less open than the current state may bebased on one or more different conditions or may be unconditional—thatis always performed. One possible implementation of the operation 106and the operation 110 is further described in connection with FIG. 8hereinafter. As in the case of operation 106, either or both of thestages 112 and 114 could be executed through a monotonic change, withlocal reversals between the respective states, and/or in a stepwisefashion, just to name representative alternatives. Further, it should beappreciated that a target state at the most open end of the adjustmentrange would require different processing because there is no ability toprovide a higher interim state, and/or the range available for selectionis defined with an upper extreme that is less than the most open statepossible to account for this aspect.

From operation 110, procedure 100 continues with an operation 120. Inoperation 120, transient reduction is performed due to possibletransients that may be generated by the multidirectional transition ofoperation 110. In particular, for system 10 in which control was basedon placement at the lower side of band B when changing from a less openposition to a more open position, repositioning at the high side of bandB can sometimes cause a transient spike to occur. Specifically, becausethe shift to the high side of band B can cause a relatively rapid dropin core speed N2 compared to low side placement, no effort may be madeto increase fuel flow to raise core speed N2 back to the previous levelfor systems based on such a pre-existing approach. In response toincreased fuel, fan speed N1 typically increases to an undesirable valuewhen engine 15 is otherwise operating with N2 near its upper limit astested by conditional 102. This undesirable fan speed N1 increase canlead to a higher than desired ITT, and/or other untoward results untilthe system adjusts to the new N2 versus N1 relationship for thecomparatively lower N2 resulting for CVG repositioning to the more openside of band B. The time it takes for this adjustment is designatedtransient time TTIM. Operation 120 is arranged to address undesiredtransient performance, as more particularly described in connection withFIG. 9 and accompanying text.

Alternative embodiments may include transient reduction thatalternatively or additionally results from operation 106. In otherembodiments, operation 120 may be performed in response to completion ofoperation 106 or operation 110 only when certain conditions aresatisfied, operation 120 may be optional, or transientreduction/elimination may be absent. In further embodiments, it may bedesirable some or all of the time to maintain CVG position toward thelow side of band B instead of the high side.

FIGS. 7-9 depict control flow diagrams describing one implementation ofprocedure 100 performed with system 10. Generally, the operationsdescribed in these figures are implemented in control logic 62 ofcontroller 50 as software programming, firmware, and/or dedicatedhardware, or the like. Correspondingly, logic 200, 300, and 400, asrespectively depicted in FIGS. 7-9, is repetitively executed on aperiodic or aperiodic basis.

More specifically, FIG. 7 depicts CVG position tracking logic 200. Logic200 maintains a limited history of the position of the actuator 61 a totrack position of stators 36 b relative to hysteresis band B. CVGposition of mechanism 60 is determined at actuator 61 a by anappropriate sensor or other measuring means (not shown) to generate CVGposition feedback signal CVGFB, as depicted in FIG. 2 and FIG. 7. Asobtained from actuator 61 a, CVGFB does not account for hysteresisvariations—instead being represented by the medial solid line withinhand B as depicted in FIG. 5.

Logic 200 also receives as input CVGHYB which is subtracted from CVGFBby a summation operator 202 of logic 200 and added to CVGFB by asummation operator 204 of logic 200. Signal CVGHYB is representative ofthe CVG hysteresis band B. The summation operator 202 outputs signal A1(A1=CVGFB−CVGHYB), and the summation operator 204 outputs signal A2(A2=CVGFB+CVGHYB). Signal A1 is provided as an input to a “greater than”(>) comparator 206 of logic 200 along with signal B to test if A1 isgreater than B (A1>B); and signal A2 is provided as an input to a “lessthan” (<) comparator 208 of logic 200 along with signal B to test if A2is less than B (A2<B). Signal A1 is also coupled to the “true” contact Tof a logical switch 210, and signal A2 is also coupled to the “true”contact T of a logical switch 212. The switches 210 and 212 are both ofthe single pole, double throw type. The respective outputs of thecomparators 206 and 208 are discrete evaluation/result signals e1 and e2that are so distinguished by lower case letters relative to othersignals. These signals are true (affirmative) if the correspondingcomparator (> or <) is true and false (negative) otherwise. Signals e1and e2 are also respectively coupled to an evaluation input (dashedline) of the switches the 210 and 212. If the evaluation input is true,then the switch wiper W contacts the true contact T and otherwisecontacts the false contact F for each of the switches 210 and 212. Thefalse contact F of switch 210 is coupled to signal B, and the falsecontact F of switch 212 is coupled to the output of switch 210.

Signal B is output by a delay stage 214. Delay stage 214 captures theprior value of CVGPOS, which represents the CVG position determinedduring the most recent prior execution of logic 200 as output by switch212. Accordingly, delay stage 214 sets B equal to CVGPOS as determinedduring this prior execution. From logic 200, CVGPOS is determined asfollows:

-   -   (a) if A1>B [(CVGFB−CVGHYB)>prior CVGPOS], then current        position=CVGPOS=CVGFB−CVGHYB;    -   (b) if A2<B [(CVGFB+CVGHYB), prior CVGPOS], then current        position=CVGPOS=CVGFB+CVGHYB; and    -   (c) if A1≦B [(CVGFB−CVGHYB)≦prior CVGPOS], then current        position=CVGPOS=prior position=B.        In other words, if (a) A1>B, then CVG is opening (changing in        the opening direction) and is at the low side of hysteresis band        B; (b) if A2<B, then CVG is closing (changing in the closing        direction) and is at the high side of band B; and (c) if A1≦B        and A2≧B, no change has occurred so CVG is at the same side of        the hysteresis band B as for the prior execution of logic 200.

FIG. 8 depicts CVG adjustment logic 300 that is repetitively executed togenerate the signals sent to actuator 61 a for changing CVG as desired.Signal CVGREF corresponds to a nominal reference position that may beselected based on any desired criteria. Typically, CVGREF is scheduledthrough a table and/or function(s) relative to efficiency and/or engineoperating extremes, such as surge margin and the like. Logic 300 isresponsive to CVGPOS from logic 200 during stable operation andselectively generates signals to reach a target setting from a more openposition (in the closing direction) under certain conditions—such ascore speed N2 exceeding an upper threshold near its maximum desiredoperating speed.

Logic 300 includes a summation operator 302 that subtracts CVGREF fromCVGPOS and provides the result as signal X (X=CVGPOS−CVGREF). CVGREF isalso provided to a delay stage 304 to produce a delayed discrete signalCRDEL to filter-out certain transients indicative of rapidly changingoperation. The resulting delayed signal, CRDEL, is of a discrete(boolean) type, and is provided to one input of a two-input AND gate306. The other input of the gate 306 receives a discrete signalcorresponding to the collective evaluation of various engine and/orcontrol system parameters. In one example, these parameters couldinclude one or more of a requisite engine thrust mode, throttle levelTLA, N1 level, N2 level, CVG headroom, and/or other parameters asdesired, that need to be present before hysteresis adjustment isperformed. This evaluation corresponds to the conditional 102 ofprocedure 100 previously described in connection with FIG. 6. In stillother embodiments, a different parameter set is evaluated or none areevaluated. If the latter, it would dispose of the need for gate 306because this second input would always be true.

The output signal X and a hysteresis allowance parameter Y are input toa less than (<) comparator 308. Parameter Y represents a hysteresisallowance amount for comparison to X, the difference between CVGREF andCVGPOS (X=CVGPOS−CVGREF). When the output of the comparator 308 is true(X<Y), it indicates the CVG position is not at the open side of thehysteresis band B. The output of gate 306 and the output of comparator308 are input to a three-input AND gate 310. Also input to the gate 310is a discrete output of a delay timer 314 that is false for a selectedduration after each CVG adjustment caused by logic 300 to space-out CVGadjustment in response thereto. Correspondingly, the output of the gate310 is true if: the output of the gate 306 is true (parameters and timer304 outputs both true), the output of the comparator 308 is true (X<Y),and the delay timer 314 has returned to true after the selectedduration.

The output of gate 310 is input to a two-input OR gate 312 along with atest override signal. Accordingly, gate 312 outputs true if the inputfrom gate 310 is true, the test override input signal is true, or bothare true. The output of gate 312 is input to the timer 314 and pulselogic 316. If this output is true, timer 314 is reset such thatre-activation of actuator 61 a of mechanism 60 is delayed for at leastthe specified timer duration; and logic 316 sends a control signal pulseto actuator 61 a to correspondingly adjust CVG in the opening or closingdirection. A limiter 318 limits the pulse as appropriate, and outputs acorresponding CVG reference addend, REFADD when further CVG actuation isneeded to approach a target position in a closing direction.

FIG. 9 depicts CVG transient reduction logic 400. Logic 400 is directedto an embodiment in which fuel control runs to a reference core speed N2that is determined with an integrator to maintain fan speed N1 at ascheduled value. For this approach, core speed N2 typically drops whenadjusting CVG from the low side to the high side of hysteresis band B.This N2 drop usually results in a temporary fueling increase that maycause fan speed N1 to increase an undesirable amount and/or excessiveITT temperature spiking. Correspondingly, a transient time period lapsesbefore adjusting to a new relationship between N1 and N2 for the CVGshift to the high side.

Logic 400 implements two switch-selectable alternative approaches toaddressing this type of transient performance. Logic 400 includes alogical switch 402 of the single pole, double throw type. Switch 402 hasa false contact F set to zero (false), and a true contact Tcorresponding to a CVG hysteresis pulse. Selection among the contacts ismade by discrete input CVTASW, which activates use of logic 400 toaddress transients when true (CVTASW=1), so that the CVG pulse isoutput, and disables transient reduction logic 400 when false (CVTASW=0)by the output of zero. A nonzero output of switch 402 resets a timer 404to time a period during which logic 400 will be active until timer 404is reset again. Timer 404 outputs a discrete ‘true’ signal during theactivation period to each of two different two-input AND gates 406 and410. Also input to gate 410 is a transient reduction mode signal TMOD,which is used to select the mode of transient reduction. TMOD is alsoinput to an inverter 408 which outputs the inverted TMOD signal that isprovided to the other input of gate 406. If TMOD is true (TMOD=1), andtransient reduction is active (output of timer 404 is true=1), then theoutput of gate 406 is true (logic level 1). Conversely, by operation ofinverter 408, the output of gate 410 is false (logic level 0). If TMODis false (TMOD=0), and transient reduction is active (output of timer404 is true=1), then the situation is reversed with the output of gate406 being false (logic level 0), and the output of gate 410 being true(logic level 1). It follows that at most only one of gates 406 and 410will output true at a given time. Accordingly, a true output from gate406 corresponds to a first reduction mode, FIRST and a true output fromgate 410 corresponds to a second reduction mode, SECOND.

Logic 400 also includes a delay stage 420 to output N2LAST, the value ofN2 from the most recent prior execution of logic 400. N2LAST is providedto a positive input of a summation operator 422 and N2 is provided to anegative input of the summation operator 422. Summation operator 422generates output DELN2=N2LAST−N2. DELN2 is input to an amplifier 424 toprovide a desired gain level and then limited by a limiter 426 of logic400.

The output of limiter 426 is input to the true contact (T) of a logicswitch 432. Logic switch 432 corresponds to a single pole, double throwtype of the kind previously described in connection with logic 200 ofFIG. 7. The false contact (F) of switch 432 is coupled to a constantzero input. The conditional input to switch 432 is the output of thegate 410 designated as signal SECOND. The output of switch 432 isprovided to a positive input of a summation operator 434. The derivativeof N2 is N2DOT, which is provided to another positive input of thesummation operator 434. The output of the operator 434 is input to anintegrator 436 to generate N2REF. N2REF is provided to a positive inputof summation operator 438. A negative input of the operator 438 isprovided by the output of a “last value hold” switch 430. Switch 430represents a “hold last value” operator that outputs the current valueof N2 when signal FIRST=0, and outputs a previous value as a heldconstant when signal FIRST=1. Summation operator 438 provides signalN2ERR, the difference between NREF and N2, which adjusts fueling.

The conditional discrete inputs to switches 430 and 432 are coupled tothe outputs of the gates 406 and 410, respectively. Accordingly, ifTMOD=1 while reduction is active, switch 430 is closed in response to atrue output of gate 406, and the zero constant of contact F is routed tothe operator 434 through the switch 432 in response to a false outputfrom gate 410. For the first reduction mode (TMOD=1), the level of N2feedback is arrested for a specified time period while N2 changes withCVG repositioning. As implemented in logic 400, the opening of gate 430provides a constant N2 feedback to summation operator 438 to besubtracted from N2ERR (N2ERR=N2REF−N2). This approach prevents the N2ERRsignal output from operator 438 from reacting to a drop in N2 initiatedby the CVG hysteresis control action. When the output of gate 406becomes false, as by expiration of the activation duration of timer 404,integrator 436 is reset, and provides for smooth transition back to theuse of current N2 feedback through operator 430. Further, the N1 primarycontrol loop may be modified in terms of gain or otherwise while thesecondary N2 control loop is arrested in this manner (switch 430closed).

In the second reduction mode (TMOD=0), input to the integrator 436 isadjusted via operator 434 to drive the integrator output N2REF down tocorrespond to the observed N2 over the time it takes for N2 speed torespond to CVG repositioning. Accordingly, DELN2 as modified byamplifier 424 and limiter 426, are added to N2DOT to provide theresulting sum to integrator 436. The resulting N2REF signal has beenadjusted by a magnitude corresponding to the effect of CVG hysteresiscontrol action on N2 so that the N2ERR signal calculated by operator 438does not react too strongly.

Many different embodiments are envisioned, for example, logic 200, 300,and/or 400 can be adapted to other control systems as desired todesirably approach a target CVG position or setting from a commondirection some or all of the time and/or with or without transientreduction. In an alternative embodiment of the present application, moreor fewer gas turbine engines are utilized with or without aircraft.Also, one or more engines may not include a high bypass fan stage. Inone form, Engine Pressure Ratio (EPR) is used in addition to or in lieuof fan speed (N1) as the primary feedback signal. In still otherembodiments a different engine control strategy, different feedbackparameters, and the like may be used with or without N1 feedbackcontrol. Furthermore, in another embodiment, the gas turbine engine is aturboshaft engine which powers another device via a mechanical linkageso that no appreciable thrust results for propulsion.

It is also envisioned that the present invention may be used with gasturbine engines having different arrangements of rotating members, suchas those including more or fewer compressors, turbines, or spools.Alternatively or additionally, more or fewer stators 36 b arereconfigurable for CVG, some or all of blades 36 a are pivotable toprovide CVG, and/or a different type of CVG mechanism is used. In stillother embodiments, CVG according to the present invention is used in adifferent application besides a gas turbine engine. In yet otherembodiments, variable geometry vanes operate in accordance with thehysteresis accommodation techniques described herein in other devicesbesides a compressor—such as a turbine or the like.

Another embodiment of the present application includes a gas turbineengine with a variable geometry mechanism including a number of vanes.This mechanism is operated to vary working fluid flow area between thevanes over a range extending from a first extreme to a second extreme.One of the extremes defines a greater flow area than another of theseextremes. Within this range, a desired setting is selected that isdifferent from the current setting, such that the desired setting iscloser to the first extreme than the current setting. To controlhysteresis of the variable geometry mechanism relative to a transitionfrom the current setting to the desired setting, the variable geometrymechanism is adjusted from the current setting to an interim statecloser to the first extreme than the desired setting, and subsequentlyfrom the interim state to the desired setting.

A further embodiment of the present application includes: operating acompressor with a working fluid passage that includes a variablegeometry mechanism operable over a range from a least open state to amost open state relative to the working fluid passage, selecting adesired setting of the mechanism that is more open than the currentsetting, and controlling hysteresis of the mechanism by opening suchmechanism from the current setting to a more open setting than thedesired setting and closing such mechanism from the more open setting tothe desired setting.

Still a further embodiment of the present application includes:operating a gas turbine engine including a compressor with a variablegeometry mechanism that has a number of vanes operable to vary flow areafor working fluid passing between the vanes, selecting a desired settingof the variable geometry mechanism different than the current setting tochange the flow area, and controlling hysteresis of the mechanism by oneof increasing or decreasing the flow area from the current setting to aninterim state followed by another of increasing or decreasing the flowarea from the interim state to the desired setting.

Yet a further embodiment of the present application includes a gasturbine engine with a variable geometry mechanism. This mechanism has anumber of vanes and is operable to vary working fluid flow area betweenthe vanes over a range extending from a first extreme to a secondextreme. One of these extreme defines a greater flow area than theother. Also included is an input device coupled to a gas turbine enginethat is operable to select a desired setting of the mechanism differentfrom the current setting. This desired setting is closer to the firstextreme than the current setting. Further included is a controllerresponsive to the input device to generate one or more control signalsto control hysteresis of the variable geometry mechanism and to whichthe variable geometry mechanism is responsive to change from the currentsetting to an interim state closer to the first extreme than the desiredsetting and subsequently to change from the interim state to the desiredsetting.

Another embodiment of the present invention includes logic executable bya controller to control operation of a gas turbine engine including acompressor with variable geometry vanes. The vanes are adjustable over arange from a least open state to a most open state relative to a workingfluid flow area between the vanes. This logic is operable to controlmechanical hysteresis of vane adjustment in response to an input tochange the vanes from a current setting to a desired setting by openingthe vanes from a current setting to an interim state and closing thevanes from an interim state to the desired setting. This desired settingis more open than the current setting and less open than the interimstate.

In still another embodiment, a gas turbine engine includes a compressorwith a variable geometry mechanism that operates to vary an amount offlow area for a working fluid passing through the compressor. Alsoincluded are means for selecting a target setting of the mechanismdifferent than the current setting to change the amount of flow area andmeans for determining if the target setting has a greater flow area orlesser flow area than the current setting and for controlling operationof the mechanism within the hysteresis band by one of increasing ordecreasing the flow area from the current setting to an interim statefollowed by another of increasing or decreasing the flow area from theinterim state to the desired setting.

A further embodiment includes: a gas turbine engine including acompressor, the compressor including a variable geometry mechanism thatoperates to vary an amount of flow area for a working fluid passingthrough the compressor; means for selecting a target state of thevariable geometry mechanism different than a current state of thevariable geometry mechanism to change the amount of flow area, thevariable geometry mechanism having a hysteresis band corresponding to adifference between reaching the target state by increasing or reachingthe target state by decreasing the amount of flow area; and means fordetermining if the target state has a greater flow area or a lesser flowarea than the current state and for controlling operation of thevariable geometry mechanism within the hysteresis band by one ofincreasing or decreasing the flow area from the current state to aninterim state followed by another of increasing or decreasing the flowarea from the interim state to the desired state.

All publications, patent, and patent applications cited in thisspecification are herein incorporated by reference as if each individualpublication, patent, or patent application were specifically andindividually indicated to be incorporated by reference and set forth inits entirety herein. Any theory of operation or finding described hereinis merely intended to enhance understanding of the present invention andshould not be construed to limit the scope of the present invention asdefined by the claims that follow to any stated theory or finding. Whilethe invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly selected embodiments have been shown and described and that allchanges, modifications, and equivalents that come within the spirit ofthe invention as defined herein or by the following claims are desiredto be protected.

1-17. (canceled)
 18. A system, comprising: a gas turbine engineincluding a variable geometry mechanism with a number of vanes, thevariable geometry mechanism being operable to vary working fluid flowarea between the vanes over a range extending from a first extreme to asecond extreme, one of the first extreme and the second extreme defininga greater flow area than another of the first extreme and the secondextreme; an input device coupled to the gas turbine engine that isoperable to select a desired state of the variable geometry mechanismdifferent than a current state of the variable geometry mechanism, thedesired state being closer to the first extreme than the current state;a controller responsive to the input device to generate one of morecontrol signals to control hysteresis of the variable geometrymechanism; and wherein the variable geometry mechanism is responsive tothe control signals to change from the current state to an interim statecloser to the first extreme than the desired state and subsequently tochange from the interim state to the desired state.
 19. The system ofclaim 18, wherein the vanes are of a stator type.
 20. The system ofclaim 18, further comprising means for determining if the desired statecorresponds to a flow area greater than or less than the current state.21. The system of claim 18, wherein the input device includes anoperator throttle control, and further comprising an aircraft, the gasturbine engine being operable to propel the aircraft and selectivelyadjust the variable geometry mechanism in response to the throttlecontrol.
 22. The system of claim 18, wherein the gas turbine engineincludes a first compressor connected to rotate with a first turbine,the first compressor including the variable geometry mechanism.
 23. Thesystem of claim 18, wherein the variable geometry mechanism isstructured with the first extreme corresponding to a maximum flow area,the second extreme corresponding to a minimum flow area, the interimstate corresponding to a flow area greater than the desired state, andthe desired state corresponding to a flow area greater than the currentstate.